Many commercial satellites are designed to provide multiple narrow beams with required isolation between beams to provide interference free communication services. Some of these multi-beam satellite systems route the input signal along a large number of routes corresponding to a large number of feed elements and set the beamweights (both amplitude and phase) along each route for forming the required beams. Multiple beams are also received by the large number of feed elements and the phase and the amplitude of the signals received by each feed element are adjusted before the signals are combined and forwarded.
Establishing and maintaining the required relative phase and amplitude between the signals for the different feed elements requires calibration of the paths of the signals through the satellite payload to take out any phase and amplitude offsets between the different signals for the different beams prior to applying the beam weights. Equipment used for frequency translation, filtering and amplification, and cables in the transmission path giving time delays are major sources of amplitude and phase differentials in the transmission paths. Even though the system is calibrated before launch, age and temperature differences can cause further amplitude and phase differentials in the transmission paths. The satellite system therefore needs to be calibrated in situ from time to time.
A number of prior art systems have been developed for calibrating the system in situ but many of these systems do not provide satisfactory calibration. For example, the prior art systems do not provide calibration for all frequencies in the system operational bandwidth. To ensure that the required beams are always achieved, it is also important the correct calibration can be applied at any frequency within the operating frequency range of the satellite system.
The invention was made in this context.